The ruling was immediately condemned by women's rights group.
Unemployed Keith Samuels, from Northampton, was given nine life sentences at Nottingham Crown Court last week for rape and robbery.
10-year hunt
The judge, Mr Justice Tucker, made a recommendation at the time that Samuels should serve at least 18 years before being considered for parole.
But on Friday he said he had not taken into account time Samuels had spent on remand.
Mr Justice Tucker said: "I propose to vary the recommendation from 18 years to 12 years."
Samuels, 35, a father-of-two, carried out a string of rapes in Northamptonshire and the Thames Valley area between 1984 and 1990.
Protection vital
His crimes went undetected for almost a decade until he was finally trapped by advances in DNA technology.
Sarah Maguire, chair of the London-based Rights of Women legal advice centre, said: "Twelve years is not enough. Women need to be protected against men like this and he should be kept in jail for as long as humanly possible."
She welcomed a current review by the government of sexual offences law, but said cases like Samuels' highlighted the need for change.
"It is very important that consideration of women's lives is not given took low a premium.
"Let us hope that that this recommendation is the very minimum."
Northern Ireland
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Serial rapist jailed for life
(14 Apr 99 | UK)
Rape victims protection comes closer to law
(03 Dec 98 | UK Politics)
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